Question

我正在玩C ++中的atomic implementation。我在上面提到的SO帖子中有以下代码摘录：

#include <atomic>
#include <cstdint>
#include <iostream>

template <class value_t, class block_t> struct my_atomic {
  using value_type = value_t;
  using difference_type = value_type;

  my_atomic() noexcept = default;
  constexpr my_atomic(value_t desired) noexcept
      : value(reinterpret_cast<block_t &>(desired)) {}
  my_atomic(const my_atomic &) = delete;

  my_atomic &operator=(const my_atomic &) = delete;
  my_atomic &operator=(const my_atomic &) volatile = delete;

  operator value_t() const noexcept {
    const block_t value = this->value;
    return reinterpret_cast<const value_t &>(value);
  }
  operator value_t() const volatile noexcept {
    const block_t value = this->value;
    return reinterpret_cast<const value_t &>(value);
  }

private:
  std::atomic<block_t> value;
};

using my_atomic_float = my_atomic<float, std::uint32_t>;

int main() {
  my_atomic_float var{5};
  float result = 5 * var;
  std::cout << "result: " << result << '\n';
  return 0;
}

当我尝试使用-Wall -Wpedantic -Werror和g++使用clang++进行编译时，clang++会编译代码，而g++则不会。好吧，当然，我可以关闭-Werror开关，但我想了解上述代码是否有危险。

我已经阅读了几篇帖子（[1]，[2]和其他一些帖子，例如[3]），但仍然不知道

如果我应该忽略/禁用警告，或者我应该重写它，
如果我要改写它，我应该如何改变？例如，我尝试过一个中间版reinterpret_cast<char &>，它也不起作用。

我将不胜感激任何帮助。谢谢！

编辑。很抱歉不共享以下内容：

~> g++ --version
g++ (GCC) 7.3.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

~> g++ -Wall -Wpedantic -Werror -std=c++14 -O3 question.cpp -o question.out
question.cpp: In instantiation of ‘constexpr my_atomic<value_t, block_t>::my_atomic(value_t) [with value_t = float; block_t = unsigned int]’:
question.cpp:33:24:   required from here
question.cpp:11:15: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
       : value(reinterpret_cast<block_t &>(desired)) {}
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
question.cpp: In instantiation of ‘my_atomic<value_t, block_t>::operator value_t() const [with value_t = float; block_t = unsigned int]’:
question.cpp:34:22:   required from here
question.cpp:19:12: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
     return reinterpret_cast<const value_t &>(value);
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors

~> clang++ --version
clang version 5.0.1 (tags/RELEASE_501/final)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

解除引用类型惩罚指针

0 个答案: